Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income	$ 17,285	$ 19,976
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	4,455	4,223
Deferred income taxes	(235)	1,009
Stock-based compensation	917	903
Net change in unrealized gains and losses on investments	1,197	0
Net change in accrued interest, premiums, and discounts on investments	(125)	(82)
Other	3	(2)
Total adjustments	23,497	26,027
Changes in operating assets and liabilities:
Accounts receivable	(1,193)	(2,524)
Inventories	(3,149)	(950)
Prepaid expenses	280	(2,841)
Other non-current assets	(90)	81
Accounts payable and accrued liabilities	862	(552)
Accrued income and other taxes	(153)	862
Other non-current liabilities	859	39
Net Cash Provided by (Used in) Operating Activities, Continuing Operations, Total	20,913	20,142
Cash flows from investing activities:
Property, plant and equipment additions	(7,598)	(5,422)
Purchase of investments	(26,887)	(21,911)
Proceeds from maturities of investments	24,035	19,000
Net Cash Provided by (Used in) Investing Activities, Continuing Operations, Total	(10,450)	(8,333)
Cash flows from financing activities:
Shares tendered for employees’ withholding taxes on stock-based compensation	(90)	(7,735)
Dividends paid	(4,446)	(3,873)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations, Total	(4,536)	(11,608)
Net change in cash and cash equivalents	5,927	201
Cash and cash equivalents at beginning of period	30,136	20,022
Cash and cash equivalents at end of period	36,063	20,223
Cash paid for:
Income taxes	5,592	2,295
Non-cash financing activities:
Non-cash effect of stock option exercises	$ 0	$ 10,237